different struct

[Jim Johnson]

what's the difference when declare a struct with typedef or NO
typedef?

=============== =============== ==
typedef struct {

.... etc

} SetupRecord;

=============== =============== ==
struct {

.... etc

} SetupRecord;

[Ian Collins]

Re: different struct

Jim Johnson wrote:

what's the difference when declare a struct with typedef or NO
typedef?
>
=============== =============== ==
typedef struct {
>
.... etc
>
} SetupRecord;
>

This defines a type in C.

=============== =============== ==
struct {
>
.... etc
>
} SetupRecord;

this does not.

Only use the first form if you are sharing types between C and C++.

The second form is unusual, use

struct SetupRecord {...};

--
Ian Collins.

[Jim Johnson]

Re: different struct

So there are 2 ways to declare a struct

1) C way of declaring

typedef struct {
>
.... etc
>
} SetupRecord;

2) C++ way of declaring a struct

struct SetupRecord {
>
.... etc
>
} ;

may I correct?

and also the following is incorrect synatx to declare a struct

struct {
>
.... etc
>
} SetupRecord;

[Jack Klein]

Re: different struct

On Mon, 10 Mar 2008 21:26:12 -0800, Jim Johnson <aopiyy001@yaho o.com>
wrote in comp.lang.c++:

So there are 2 ways to declare a struct
>
1) C way of declaring

No, this is not declaring anything, this is defining a type, and also
creating an alias for it.

typedef struct {

.... etc

} SetupRecord;

This is also perfectly valid C++. It defines a structure type with no
name, and defines SetupRecord as an alias for that type. This is
actually not a particularly good idea in either language.

2) C++ way of declaring a struct

Again, your sample below does not declare a struct, it defines a
struct type.

struct SetupRecord {

.... etc

} ;

This is perfectly valid C does as well. In both languages, it defines
a structure type named SetupRecord. In either language, the type can
be referred to by the two-word phrase "struct SetupRecord". In C++
only, it can also be referred to merely as SetupRecord.

may I correct?
>
and also the following is incorrect synatx to declare a struct

Your terminology is still incorrect. Your snippet below defines a
structure type, and also defines an object of that structure type.

struct {

.... etc

} SetupRecord;

You have defined an unnamed structure type, and an instance of that
type named SetupRecord. In this situation, SetupRecord is the name of
a single object of this type, and not of the structure type itself.

It is perfectly valid code, and this object can be accessed anywhere
within its scope. Compile and execute the following example:

#include <iostream>

struct { int x; int y; } ss;

void show_me()
{
std::cout << ss.x << " + " << ss.y << " = "
<< (ss.x + ss.y) << std::endl;
}

int main()
{
ss.x = 2;
ss.y = 3;
show_me();
return 0;
}

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html

[Tim Slattery]

Re: different struct

Jim Johnson <aopiyy001@yaho o.comwrote:

>what's the difference when declare a struct with typedef or NO
>typedef?
>
>============== =============== ===
>typedef struct {
>
.... etc
>
>} SetupRecord;

This defines SetupRecord to be a type, which corresponds to this
structure. You can define other items of type SetupRecord:

SetupRecord xyz;

"xyz" is now a variable of type SetupRecord.

>============== =============== ===
>struct {
>
.... etc
>
>} SetupRecord;

This defines a variable named SetupRecord. It's a structure whose
properties you've just defined.

--
Tim Slattery
Slattery_T@bls. gov

http://members.cox.net/slatteryt

[Ron Natalie]

Re: different struct

Ian Collins wrote:

Jim Johnson wrote:

>what's the difference when declare a struct with typedef or NO
>typedef?
>>
>============== =============== ===
>typedef struct {
>>
> .... etc
>>
>} SetupRecord;
>>

This defines a type in C.
>

>============== =============== ===
>struct {
>>
> .... etc
>>
>} SetupRecord;

>
this does not.
>
Only use the first form if you are sharing types between C and C++.
>
The second form is unusual, use
>
struct SetupRecord {...};
>

The latter doesn't define a type at all, it defines a variable of
unnamed type called SetupRecord.